1. Field of the Invention
The invention relates to compatible polymer mixtures comprised of polymethacrylate esters and styrene-acrylate copolymers.
2. Discussion of the Background
When polymers are mixed there is an overwhelming likelihood that they will be incompatible (see Kirk-Othmer, "Encyclopedia of Chemical Technology", 1982, Vol. 18, 3rd Ed., pub. John Wiley, pp. 443-478). Polystyrene and polyalkyl methacrylates are good examples of incompatible polymers. Thus, M. T. Shaw et al. (see Chem. Abstr. 101:73417e) reported a miscibility of only 3.4 ppm polymethyl methacrylate (PMMA, m.w. 1600,000) with polystyrene. Even very low molecular weight polystyrene is relatively incompatible with PMMA (Parent, R. R., et al., J. Polym. Sci., Polym. Phys. Ed., 16:1829, 1987). Other polyalkyl methacrylates are also incompatible with polystyrene. Apparently the only exceptions are mixtures of polystyrene and polycyclohexyl acrylate and polycyclohexyl methacrylate (see Ger. OS No. 36 32 369). Thus, styrene homopolymer is incompatible with nearly all polyalkyl methacrylates, but this incompatibility does not apply in the case of copolymers of styrene and acrylonitrile. Thus, compatibility has been found between certain styrene/acrylonitrile copolymers and PMMA (see Barlow, J. W. et al., Polymer, 28:1177, 1987). However, because this compatibility has apparently been found only for very narrowly specified copolymer compositions of the styrene/acrylonitrile component, the compatibility situation is described as having "miscibility windows". In such cases, the miscibility of the specific styrene/acrylonitrile copolymers with PMMA can be attributed to substantial repulsive forces between the styrene unit and the acrylonitrile unit in the copolymer.
A similar condition is believed to prevail in the case of styrene/maleic anhydride copolymers, which are compatible with PMMA at certain ratios of styrene t maleic anhydride. This group of PMMA-compatible styrene copolymers probably includes copolymers of styrene and allyl alcohol and copolymers of styrene and p-(2-hydroxyhexafluoroisopropyl)styrene, the PMMA compatibility of which has been interpreted as being due to hydrogen bonding of the hydroxyl group with the PMMA ester group (Min, B. Y., and Pearce, Eli M., Org. Coatings and Plast. Chem., 45:45:58-64, 1981; Cangelori, F., and Shaw, M. T., 1983 Polymer Reprints (Am. Chem. Soc., Div. Polym. Chem.), 24:258-259, 1983).
Although compatibility of PMMA with copolymers of styrene and strongly polar monomers such as acrylonitrile, maleic anhydride, allyl alcohol, and p-(2-hydroxyhexafluoroisopropyl)styrene has been known for a number of years, these compatible polymer mixtures have been and continue to be regarded as special exceptions in the large range of incompatible polystyrene/polyalkyl methacrylate mixtures. This view is particularly understandable in that as a rule the compatibility of these styrene copolymers has been limited to PMMA as a mixture partner.
Also, for a long time polyalkyl methacrylates and polyalkyl acrylates have been regarded as mutually incompatible. However, as demonstrated in unpublished Ger. Pat. App. P No. 37 08 427.5 of Mar. 16, 1987, under certain conditions polyalkyl methacrylates and polyalkyl acrylates are compatible. The prerequisites for good compatibility, according to P No. 37 08 427.5 are:
a) Comparability of the Van der Waals volumes of the alkyl groups of the polymethacrylate and the polyacrylate; and
b) Alkyl groups which are large (5-40 carbon atoms) and are sterically hindering and spatially fixed.
Examples which might be mentioned of mixtures which are compatible in any ratio, even at room temperature, are polycyclohexyl acrylate/polycyclohexyl methacrylate, and poly-3,3,5-trimethylcyclohexyl acrylate/poly-3,3,5-trimethylcyclohexyl methacrylate.
As long as the number of carbon atoms in the alkyl groups of the polyacrylate and polymethacrylate coincide, compatibility has been found at higher temperatures even for short-chain alkyl groups. Thus, for the system polyethyl acrylate/polyethyl methacrylate, complete miscibility has been found for temperatures above 190.degree. C.; also for the system polybutyl acrylate/polybutyl methacrylate (see FIGS. 3 and 4 of Ger. Pat. App. P No. 37 08 427.5). Such mixtures of polyalkyl acrylate and polyalkyl methacrylate thus display an upper critical solution temperature (UCST).
In certain instances and in certain areas of the plastics industry, mechanical mixtures of polymers have led to products with improved properties, and in some cases the range of possible applications of the subject materials has been substantially broadened (see Kirk-Othmer, loc. cit., Vol. 18). The physical properties of such "polyblends" ordinarily represent a compromise; in favorable cases, such compromises can furnish a net improvement in comparison with the properties of the individual polymers. On the other hand, the technical applicability of incompatible (multiphase) polymer mixtures is often evaluated as better than that of compatible mixtures (see Kirk-Othmer, loc. cit., P. 449).
Compatible polymer mixtures with favorable mechanical properties can be regarded as technically useful based particularly on the fact that they are expected to have good optical properties. On the other hand, based on the known state of the art, there is little prospect of obtaining technically useful polymer mixtures from components having as their respective components styrene (co)polymers, and polyalkyl(meth)acrylates, particularly if it is required that the polymer mixtures be compatible at room temperature at all mixture ratios.